The present invention relates to electrophotographic image forming apparatuses such as copying machines, laser printers, and facsimile machines, and more specifically relates to image forming apparatuses of a mode in which a surface of a photosensitive drum (electrostatic latent bearing member) is charged by a charging unit such as a corona charging unit.
As electrophotographic image forming apparatuses such as copying machines, there are monochrome image forming apparatuses that form black and white images, and color image forming apparatuses that form color images. As color image forming apparatuses there are image forming apparatuses of a multiple rotation mode in which toner images of each color component are formed in order on a single photosensitive body by way of toner image formation means for each color (black, cyan, magenta, and yellow) as disclosed in Japanese Patent Application No. 2003-191526 for example, and image forming apparatuses of a tandem mode in which a plurality of toner image formation means, which form toner images of the color components approximately simultaneously on separate respective photosensitive bodies, are arranged serially along a carrying direction of an intermediate transfer material.
On the other hand, corona charging units such as a corotron charging unit using a wire and a case as disclosed in Japanese Patent Application No. 2002-229302 for example, and a scorotron charging unit that uses a wire (discharging electrode), a case, and a grid electrode to stabilize an electric potential on the surface of a photosensitive body, are widely used as charging units for electrophotographic image forming apparatuses. In particular, scorotron charging units offer the advantage of being able to stably control the electric potential on the surface of a photosensitive body by using a grid arranged between the discharging electrode and the surface of the photosensitive body.
In this regard, ozone is produced inside the charging unit when using a corona charging unit as the charging unit, which presents a problem of image deterioration when the produced ozone becomes residual and adversely affects the surface of the photosensitive drum. Furthermore, there is a problem that developer spatters during the development process and the inside of the apparatus becomes soiled by the spattered developer. Conventionally, in order to solve such problems, systems are employed in which an exhaust fan or the like is used to provide centralized ventilation within the apparatus main unit.
However, with conventional ventilation systems, air in the vicinity of the fixing device is also drawn so that the heat produced by the fixing device exerts an adverse influence on the image formation process. For example, toner blocking, thermal deformation of the apparatus housing and other problems occur. To avoid this influence, it has been necessary to install the fixing device and the portion for image formation processing apart from each other at a fixed distance, which presents an obstacle to miniaturization of the apparatus. Furthermore, when carrying out ventilation for the entire apparatus including the vicinity of the fixing device, it has been necessary to use a large capacity exhaust fan and a dedicated duct, which also presents an obstacle to miniaturization.
Further still, with centralized ventilation systems, the ventilation within the charging units installed in the portion for image formation processing is incomplete, which makes it impossible to avoid image deterioration and reduced life of the image formation process due to such factors as ozone damage to the photosensitive drums and nitrogen oxides adhering to the grid of the charging units.